There is a high demand for a reduction in size, weight, and cost of base stations in mobile communication systems. The base stations use a transmitting filter for transmission and a receiving filter for reception during transmission and reception of radio signals in order to reduce undesired and unnecessary radio frequency waves. These transmitting and receiving filters are band-pass filters and may be collectively called a “filter” in the following description.
The insertion loss in the pass-band in each filter mainly causes degradation of power efficiency in the transmitting filter and causes degradation of noise figure (NF) in the receiving filter. For this reason, it is necessary to minimize the insertion loss in the pass-band in the filters. In order to minimize the insertion loss in the pass-band, high unloaded Q is required in the resonator.
In addition, the weight of the filters occupies about 30% of the weight of the entire base station, and thus has a large influence on the weight of the apparatus.
When a general TEM (Transverse Electro Magnetic) mode air-cavity filter is used, an increase in unloaded Q requires an increase in size of the filter structure, which conflicts with a desired reduction in size and weight. Meanwhile, use of a dielectric filter allows for reduction in size and weight but causes an increase in cost. In this respect, Patent Literature (hereinafter, referred to as “PTL”) 1 discloses a semi-coaxial resonator used in a filter that achieves a reduction in size, weight, and cost. Note that, the term “semi-coaxial” refers to a coaxial line having a short-circuited end.
In the resonator disclosed in PTL 1 (λ/4 air semi-coaxial resonator), the casing (outer conductor) is formed in a box shape, and the open end of a resonator body (inner conductor) housed in the casing is formed in a disk shape, thereby achieving low impedance for wavelength shortening. Thus, a reduction in the height of resonator body and casing (capacity reduction) is achieved.
In general, multiple resonator bodies are used. Thus, FIG. 1 illustrates a state where two resonator bodies are used in the resonator disclosed in PTL 1. As illustrated in FIG. 1, the walls inwardly protruding respectively from side surfaces of the casing are provided between two resonator bodies for the purpose of reducing the electromagnetic field coupling between the two resonator bodies.